In recent years, large-capacity (Wh) secondary batteries have been developed as power sources of hybrid electric vehicles or pure electric vehicles and among others, square lithium ion secondary batteries with a high energy density (Wh/kg) have attracted attention.
In a square lithium ion secondary battery, a flat wound electrode group is formed as a power generation element by stacking up and winding a positive electrode in which positive electrode foil is coated with a positive electrode active material, a negative electrode in which negative electrode foil is coated with a negative electrode active material, and a separator to insulate the positive electrode and the negative electrode. The wound electrode group is electrically connected to a positive electrode external terminal and a negative electrode external terminal provided on a battery lid of a battery container. The wound electrode group is housed in a battery can of the battery container and an opening of the battery can is sealed with the battery lid and welded. A square secondary battery is formed by injecting an electrolytic solution through an injection hole of the battery container and then blocking off the injected solution by an injection tap and sealing by laser welding.
A battery module is formed by electrically connecting the positive electrode external terminal and the negative electrode external terminal of a plurality of square secondary batteries using a conductive member such as a bus bar. The bus bar is connected to the square secondary battery by being screwed onto an external terminal using a bolt and a nut or welded to the external terminal.
In PTL 1, an external terminal connecting a plurality of secondary batteries and formed from a clad material made of aluminum and nickel and a connection terminal (electrode led pin in PTL 1) made of aluminum from inside the secondary battery are provided. The secondary battery described in PTL 1 is fixed by caulking to the connection terminal joined with a power generation element on the nickel side of the external terminal.
In PTL 2, a plurality of square secondary batteries is connected by a bus bar being welded to the external terminals, each external terminal to be connected is made of an alloy of aluminum and copper, and the bus bar includes a clad material in which an aluminum alloy and a copper alloy are joined as dissimilar metals. The bus bar described in PTL 2 has an aluminum alloy and a copper alloy are butt-joined on narrow-width surfaces and the aluminum alloy of the bus bar and the connection terminal made of the aluminum alloy are welded to the copper alloy of the bus bar and the connection terminal made of the copper alloy for connection.